Search engine guru Greg Notess has long studied search engine overlap -- the number of pages found by more than one search engine. Greg's findings have consistently shown that there is very little overlap in the web page databases of the major search engines, meaning you'll likely get very different results depending on the engine.

The new Thumbshots Ranking Tool allows you to perform your own comparisons, and displays the results visually, making it easy to see both the rankings and comparative positions of pages in search engine results. URLs are represented by small circles, and these circles are connected by a line if the page appears in both engines you're testing. Mouse over a circle and the full URL of the page is displayed.

The results for most queries confirm Greg Notess' research: there is very little overlap in search engine results for most of the tests I ran. And in most cases, even the top ten results vary significantly from engine to engine.

The ranking tool fetches the top 100 results for a query from AlltheWeb, AltaVista, Google, MSN, Teoma, Wisenut or Yahoo. You can compare the same search term on two engines, or two unique search phrases on the same engine to see which ranks better. You can also enter a specific website, to see if and where the site appears in the top 100 for a particular query.

While the comparative results are interesting, they aren't always exactly the same as you'll see when doing a search directly on a particular engine. According to Stephen Lim, Executive Director of Thumbshots.org (the folks who created the tool), results may vary due to your locality, or from the tool hitting different data centers.

Lim says that they've made a number of interesting observations in using the tool. For example, the majority of sites that are ranked highly in Google rank poorly in Yahoo, and vice-versa. This suggests that webmasters optimizing for Google alone may be missing significant traffic from other search engines.

He also notes that despite AlltheWeb and AltaVista now using the Yahoo database, results for the three engines still differ for the same query, though the divergence appears to be narrowing.

Lim offered a number tips for searchers, drawn from experiences using the tool. Since links from various search engines do not overlap significantly, it's important to try different search engines from time to time, for a number of reasons.

Search ranking and results vary significantly with plurality. For example, using the ranking tool to compare "business" and "businesses" within Google shows very little correlation between results.

This suggests that the ranking tool can actually help searchers improve their queries, by running simple variations at the same time and comparing results.

Search ranking and pages returned as results also changes significantly with the addition of a single related keyword. For example, comparing "freeware" and "freeware software" within Teoma yields only a 30% overlap, though interestingly, many of the top ten results are highly correlated.

One of the coolest features of the comparison tool is the addition of "thumbshots" for many of the results. These are miniature screenshots of a site. Mousing over the circles representing URLs will bring up a thumbshot for the page, if it is available.

The Thumbshots ranking tool was created by volunteers, and is free to use. It's one of those rare tools that's equally useful to webmasters and searchers alike.

About the author

Chris Sherman is a frequent contributor to several information industry journals. He's written several books, including The McGraw-Hill CD ROM Handbook and The Invisible Web: Uncovering Information Sources Search Engines Can't See, co-authored with Gary Price. Chris has written about search and search engines since 1994, when he developed online searching tutorials for several clients. From 1998 to 2001, he was About.com's Web Search Guide.